Metadata Backup and the Naming Mechanism

Metadata server holds the most critical information about striping and authorization. The failure of the metadata server will crash the whole storage system. Therefore, the metadata server needs to be backed up to improve reliability. However, the original PVFS can not achieve the backup of the metadata server due to the limitation of its naming mechanism for the striped files. In PVFS, the striped data in a data server is sieved together and stored as a file. In addition, the file name is chosen to be the inode number of the metadata file to guarantee the uniqueness of the file name in the data servers. This approach has two main disadvantages. One is that the total number of inodes on the metadata server is limited. When the file number is large, we may run out of inode numbers. The other, more significant disadvantage is that PVFS can not backup the meta server theoretically because the data of a new file will be falsely written into an existing file when the primary metadata server is down and the backup metadata server assigns the new file an inode number that has been used by the primary metadata server.

In the design of CEFT-PVFS, we have changed the naming mechanism and instead used the MD5 sum [16] of the requested file name as the data file name. In this way, the metadata can be directly duplicated to any backup storage device to provide redundancy.

The calculation of MD5 will not introduce significant overhead in CEFT-PVFS. First, we only need to calculate the MD5 of file names, which are typically 5-20 bytes. While we measured that the MD5 program can calculate with a speed of 200 MB/sec on a single node, the calculation of a file name usually takes only 25-100 $ns$. Second, the MD5 calculation is not the bottleneck since it is performed distributively by client nodes. Each client node calculates the MD5 of its destination file name and sends the result along with its I/O requests to the metadata server so that the metadata server can directly extract it from the requests.